Swedish birds are tracking temperature but not rainfall: evidence from a decade of abundance changes

Aim:To quantify avian distribution shifts and the extent of niche tracking in response to changing temperature and precipitation patterns. 

Location: Sweden. 

Methods: We used abundance monitoring data to quantify changes in bird species distributions between two time periods, 2000-02 and 2010-12. First we examined shifts at the level of whole distributions using population centroids in temperature, rainfall, altitude, latitude and longitude. We then characterized shifts in temperature and latitude at different parts of species ranges using species response curves (SRC). We accounted for yearly turnover in abundance and sampling effort, and compared the observed shifts with those expected under perfect niche tracking. 

Results: Most species demonstrated changes in their distributions over the last decade but not all were in response to weather. The degree to which species tracked their climatic niches and the dynamics driving these shifts varied considerably. Only 20% of species shifted in the direction expected given the temperature changes, while few showed a strong response to rainfall. Most shifts did not fully compensate for changes in temperature. Range changes were most evident at the leading edges, but there was some evidence for retractions of trailing edges. Amongst species that tracked temperature, those with southerly distributions were less successful at tracking changes than those in the north. 

Main conclusions: Swedish birds demonstrated highly dynamic distributions, with many rapid directional shifts occurring over the last decade. However, only a few species kept pace with observed climatic change. Species that did not track their climatic niche may be tolerant to ongoing climatic change or constrained by strong habitat requirements. We demonstrate that measuring range shifts along both environmental and geographic gradients can help disentangle drivers of distribution changes.